The present invention relates to a DC motor driving device.
An example of a DC motor driving device which receives a digital signal and effects the selective rotation of ones of DC motors in a direction specified by the content of the digital signal is shown in FIG. 1.
In FIG. 1, input terminals IN.sub.0 through IN.sub.3 are connected to a decoder 1. In the decoder 1, a four-bit digital signal is subjected to logic conversion to obtain a 12-bit logic output. The first through twelfth output terminals of the decoder 1 are connected to a switch circuit 2. The switch circuit 2 has twelve transistors 201 through 212 corresponding to the first through twelfth output terminals of the decoder 1. The bases of the transistors 201 through 212 are connected to the first through twelfth output terminals, respectively, of the decoder 1. The emitter of the transistor 201 is connected to the collector of the transistor 204, and their connecting point is connected to an output terminal O.sub.2 of the switch circuit 2. The emitter of the transistor 202 is connected to the collector of the transistor 205, and their connecting point is connected to an output terminal O.sub.1 of the switch circuit 2. The emitter of the transistor 203 is connected to the collector of the transistor 206, and their connecting point is connected to an output terminal O.sub.0 of the switch circuit 2. The collectors of the transistors 201 through 203 receive a supply voltage V.sub.cc. The emitters of the transistors 204 through 206 are grounded. The switch circuit 2 has output terminals O.sub.3 through O.sub.5. The transistors 207 through 212 are connected in the same circuit configuration as the above-described transistors 201 through 206, and are connected to the output terminals O.sub.3, O.sub.4 and O.sub.5 as shown in FIG. 1.
A DC motor 3 is connected between the output terminals O.sub.0 and O.sub.1, a DC motor 4 between the output terminals O.sub.1 and O.sub.2, a DC motor 5 between the output terminals O.sub.3 and O.sub.4, and a DC motor 6 between the output terminals O.sub.4 and O.sub.5. Each of the DC motors 3 through 6 has positive and negative terminals. The negative terminals of the motor 3 and 4 are connected to the output terminal O.sub.1, while the negative terminals of the motors 5 and 6 are connected to the output terminal O.sub.4. When the voltage V.sub.cc is applied to the motors 3 through 6 in the normal polarity, the motors rotate in a predetermined direction, here assumed to be the forward direction. When the voltage V.sub.cc is applied to the motors with the opposite polarity, the motors turn in the reverse direction.
In the conventional DC motor driving device thus constructed, a four-bit digital signal is applied through the input terminals IN.sub.0 through IN.sub.3 to the input terminals A, B, C and D of the decoder 1. In the decoder 1, the four-bit digital signal is converted into a 12-bit digital signal using a predetermined coding system, and the converted output is provided at the first through twelfth output terminals. Specifically, when the input digital signal is a logic "0000", then all the output terminals are at logic "0", and when the input digital signal is a value between logic "0001" and "1111", then one of the first through twelfth output terminals is at logic "1". The decoder 1 provides positive logic outputs; that is, logic "0" corresponds to a low level and logic "1" to a high level. Thus, only transistors in the switch circuit 2 connected to output terminals of the decoder 1 at the high level are rendered conductive (turned on). If, for instance, the third and fifth output terminals of the decoder 1 are high levels, then the transistors 203 and 205 are rendered conductive. Therefore, the voltage V.sub.cc is applied through the transistors 203 and 205 to the motor 3 in the normal polarity to cause the latter to rotate in the forward direction.
When the content of the input digital signal changes, the levels of specific ones of the output terminals also change. If, for example, the second and sixth output terminals of the decoder 1 are raised to the high level, then the transistors 202 and 206 are rendered conductive, as a result of which the voltage V.sub.cc is applied to the motor 3 in the opposite polarity to cause the latter to rotate in the reverse direction.
It should be noted that the content of the input digital signal is such that two series-connected transistors (transistors 201 and 204, for instance) cannot rendered conductive at the same time.
In the conventional DC motor driving device, the response time which elapses from the time instant that a digital signal is supplied to the decoder 1 until new conversion results are provided at the output terminals depends upon the content of the applied input digital signals. That is, the decoder 1 is implemented with a number of gate circuits, such as NAND gates and OR gates, and input digital signals pass through different numbers of gates depending on the states of the various bits thereof, as a result of which different response times can occur for different output terminal for some changes in the state of the input signal bits. Thus, although the decoder is designed so that two series-connected transistors should not be rendered conductive at the same time, due to the different response times at different the output terminals, sometimes two series-connected transistors are momentarily turned on simultaneously. In such an event, large currents flow in the two transistors, as a result of which the transistors can be damaged or their servce life reduced.